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<!doctype html public "-//w3c//dtd html 4.0 transitional//en" "http://www.w3.org/TR/REC-html40/loose.dtd"><html><head> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> <meta name="Author" content="David Turner"> <title>The design of FreeType 2</title></head><body text="#000000" bgcolor="#FFFFFF" link="#0000EF" vlink="#51188E" alink="#FF0000"><h1 align=center> The design of FreeType 2</h1><h3 align=center> Copyright 1998-2000 David Turner (<a href="mailto:david@freetype.org">david@freetype.org</a>)<br> Copyright 2000 The FreeType Development Team (<a href="mailto:devel@freetype.org">devel@freetype.org</a>)</h3><center><table width="65%"><tr><td> <center> <table width="100%" border=0 cellpadding=5> <tr bgcolor="#CCFFCC" valign=center> <td align=center width="30%"> <a href="design-4.html">Previous</a> </td> <td align=center width="30%"> <a href="index.html">Contents</a> </td> <td align=center width="30%"> <a href="design-6.html">Next</a> </td> </tr> </table> </center> <p><hr></p> <table width="100%"> <tr bgcolor="#ccccee"><td> <h1> IV. Module Classes </h1> </td></tr> </table> <p>We will now try to explain more precisely the <em>types</em> of modules that FreeType 2 is capable of managing. Note that each one of them is decribed with more details in the following chapters of this document.</p> <ul> <li> <p><em>Renderer</em> modules are used to manage scalable glyph images. This means <em>transforming</em> them, computing their <em>bounding box</em>, and <em>converting</em> them to either <em>monochrome</em> or <em>anti-aliased</em> bitmaps</em>.</p> <p>Note that FreeType 2 is capable of dealing with <em>any</em> kind of glyph images, as long as a renderer module is provided for it. The library comes by default with two renderers:</p> <p><table cellpadding=8> <tr valign=top> <td> <tt>raster</tt> </td> <td> <p>Supports the conversion of vectorial outlines (described by a <tt>FT_Outline</tt> object) to <em>monochrome</em> bitmaps. </td> </tr> <tr valign=top> <td> <tt>smooth</tt> </td> <td> <p>Supports the conversion of the same outlines to high-quality <em>anti-aliased</em> pixmaps (using 256 levels of gray). Note that this renderer also supports direct span generation.</p> </td> </tr> </table></p> <li> <p><em>Font driver</em> modules are used to support one or more specific font format. By default, FreeType 2 comes with the following font drivers:</p> <p><table cellpadding=8> <tr valign=top> <td> <tt>truetype</tt> </td> <td> <p>supports TrueType font files</p> </td> </tr> <tr valign=top> <td> <tt>type1</tt> </td> <td> <p>supports Postscript Type 1 fonts, both in binary (<tt>.pfb</tt>) or ASCII (<tt>.pfa</tt>) formats, including Multiple Master fonts.</p> </td> </tr> <tr valign=top> <td> <tt>cid</tt> </td> <td> <p>supports Postscript CID-keyed fonts</p> </td> </tr> <tr valign=top> <td> <tt>cff</tt> </td> <td> <p>supports OpenType, CFF as well as CEF fonts (CEF is a derivative of CFF used by Adobe in its SVG viewer)</p> </td> </tr> <tr valign=top> <td> <tt>winfonts</tt> </td> <td> <p>supports Windows bitmap fonts (i.e. <tt>.fon</tt> and <tt>.fnt</tt>)</p> </td> </tr> </table></p> <p>Note that font drivers can support bitmapped or scalable glyph images. A given font driver that supports Bézier outlines through <tt>FT_Outline</tt> can also provide its own hinter, or rely on FreeType's <tt>autohinter</tt> module.</p> </li> <li> <p><em>Helper</em> modules are used to hold shared code that is often used by several font drivers, or even other modules. Here are the default helpers:</p> <p><table cellpadding=8> <tr valign=top> <td> <tt>sfnt</tt> </td> <td> used to support font formats based on the <tt>SFNT</tt> storage scheme: TrueType & OpenType fonts as well as other variants (like TrueType fonts that only contain embedded bitmaps) </td> </tr> <tr valign=top> <td> <tt>psnames</tt> </td> <td> used to provide various useful functions related to glyph names ordering and Postscript encodings/charsets. For example, this module is capable of automatically synthetizing a Unicode charmap from a Type 1 glyph name dictionary. </td> </tr> <tr valign=top> <td> <tt>psaux</tt> </td> <td> used to provide various useful functions related to Type 1 charstring decoding, as this "feature" is needed by the <tt>type1</tt>, <tt>cid</tt>, and <tt>cff</tt> drivers. </td> </tr> </table></p> </li> <li> <p>Finally, the <em>autohinter</em> module has a specific role in FreeType 2, as it can be used automatically during glyph loading to process individual glyph outlines when a font driver doesn't provide it's own hinting engine.</p> <p>This module's purpose and design is also heavily described on the FreeType web site.</p> </li> </ul> <p>We will now study how modules are described, then managed by the library.</p> <h3> 1. The <tt>FT_Module_Class</tt> structure </h3> <p>As described later in this document, library initialization is performed by calling the <tt>FT_Init_FreeType()</tt> function. The latter is in charge of creating a new "empty" <tt>FT_Library</tt> object, then register each "default" module by repeatedly calling the <tt>FT_Add_Module()</tt> function.</p> <p>Similarly, client applications can call <tt>FT_Add_Module()</tt> any time they wish in order to register a new module in the library. Let us take a look at this function's declaration:</p> <font color="blue"><pre> extern FT_Error FT_Add_Module( FT_Library library, const FT_Module_Class* clazz );</pre> </font> <p>As one can see, this function expects a handle to a library object, as well as a pointer to a <tt>FT_Module_Class</tt> structure. It returns an error code. In case of success, a new module object is created and added to the library. Note by the way that the module isn't returned directly by the call!</p> <p>Here the definition of <tt>FT_Module_Class</tt>, with some explanation. The following code is taken from <tt><freetype/ftmodule.h></tt>:</p> <font color="blue"><pre> typedef struct FT_Module_Class_ { FT_ULong module_flags; FT_Int module_size; const FT_String* module_name; FT_Fixed module_version; FT_Fixed module_requires; const void* module_interface; FT_Module_Constructor module_init; FT_Module_Destructor module_done; FT_Module_Requester get_interface; } FT_Module_Class;</pre> </font> <p>A description of its fields:</p> <p><table cellpadding=8> <tr valign=top> <td> <tt>module_flags</tt> </td> <td> <p>A set of bit flags used to describe the module's category. Valid values are:</p> <ul> <li> <tt>ft_module_font_driver</tt> if the module is a font driver </li> <li> <tt>ft_module_renderer</tt> if the module is a renderer </li> <li> <tt>ft_module_hinter</tt> if the module is an auto-hinter </li> <li> <tt>ft_module_driver_scalable</tt> if the module is a font driver supporting scalable glyph formats </li> <li> <tt>ft_module_driver_no_outlines</tt> if the module is a font driver supporting scalable glyph formats that <em>cannot</em> be described by an <tt>FT_Outline</tt> object </li> <li> <tt>ft_module_driver_has_hinter</tt> if the module is a font driver that provides its own hinting scheme/algorithm </li> </ul> </td> </tr> <tr valign=top> <td> <tt>module_size</tt> </td> <td> <p>An integer that gives the size in <em>bytes</em> of a given module object. This should <em>never</em> be less than <tt>sizeof(FT_ModuleRec)</tt>, but can be more if the module needs to sub-class the base <tt>FT_ModuleRec</tt> class.</p> </td> </tr> <tr valign=top> <td> <tt>module_name</tt> </td> <td> <p>The module's internal name, coded as a simple ASCII C string. There can't be two modules with the same name registered in a given <tt>FT_Library</tt> object. However, <tt>FT_Add_Module()</tt> uses the <tt>module_version</tt> field to detect module upgrades and perform them cleanly, even at run-time.</p> </td> </tr> <tr valign=top> <td> <tt>module_version</tt> </td> <td> <p>A 16.16 fixed float number giving the module's major and minor version numbers. It is used to determine whether a module needs to be upgraded when calling <tt>FT_Add_Module()</tt>.</p> </td> </tr> <tr valign=top> <td> <tt>module_requires</tt> </td> <td> <p>A 16.16 fixed float number giving the version of FreeType 2 that is required to install this module. The default value is 0x20000 for FreeType version 2.0</p> </td> </tr> <tr valign=top> <td> <tt>module_requires</tt> </td> <td> <p>Most modules support one or more "interfaces", i.e. tables of function pointers. This field is used to point to the module's main interface, if there is one. It is a short-cut that prevents users of the module to call "get_interface()" each time they need to access one of the object's common entry points.</p> <p>Note that is is optional, and can be set to NULL. Other interfaces can also be accessed through the <tt>get_interface()</tt> field.</p> </td> </tr> <tr valign=top> <td> <tt>module_init</tt> </td> <td> <p>A pointer to a function used to initialize the fields of a fresh new <tt>FT_Module</tt> object. It is called <em>after</em> the module's base fields have been set by the library, and is generally used to initialize the fields of <tt>FT_ModuleRec</tt> subclasses.</p> <p>Most module classes set it to NULL to indicate that no extra initialization is necessary.</p> </td> </tr> <tr valign=top> <td> <tt>module_done</tt> </td> <td> <p>A pointer to a function used to finalize the fields of a given <tt>FT_Module</tt> object. Note that it is called <em>before</em> the library unsets the module's base fields, and is generally used to finalize the fields of <tt>FT_ModuleRec</tt> subclasses.</p> <p>Most module classes set it to NULL to indicate that no extra finalization is necessary</p> </td> </tr> <tr valign=top> <td> <tt>get_interface</tt> </td> <td> <p>A pointer to a function used to request the address of a given module interface. Set it to NULL if you don't need to support additional interfaces but the main one.</p> </td> </tr> </table></p> <h3> 2. The <tt>FT_Module</tt> type </h3> <p>The <tt>FT_Module</tt> type is a handle (i.e. a pointer) to a given module object/instance, whose base structure is given by the internal <tt>FT_ModuleRec</tt> type. We will intentionally <em>not</em> describe this structure here, as there is no point to look so far into the library's design.</p> <p>When <tt>FT_Add_Module</tt> is called, it first allocates a new module instance, using the <tt>module_size</tt> class field to determine its byte size. The function initializes the root <tt>FT_ModuleRec</tt> field, then calls the class-specific initializer <tt>module_init</tt> when this field is not set to NULL.</p> <p>Note that the library defines several sub-classes of <tt>FT_ModuleRec</tt>, which are, as you could have guessed:</p> <ul> <li><p><tt>FT_Renderer</tt> for renderer modules</p> <li><p><tt>FT_Driver</tt> for font driver modules</p> <li><p><tt>FT_AutoHinter</tt> for the auto-hinter</p> </ul> <p>Helper modules use the base <tt>FT_ModuleRec</tt> type. We will describe these classes in the next chapters.</p> <p><hr></p> <center> <table width="100%" border=0 cellpadding=5> <tr bgcolor="#CCFFCC" valign=center> <td align=center width="30%"> <a href="design-4.html">Previous</a> </td> <td align=center width="30%"> <a href="index.html">Contents</a> </td> <td align=center width="30%"> <a href="design-6.html">Next</a> </td> </tr> </table> </center></td></tr></table></center></body></html>⌨️ 快捷键说明
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